Electrode Site
An electrode site is one exposed patch of metal on a thin-film probe or grid — the contact that senses the extracellular voltage on its surface and sources one signal channel.
Electrode 'site' and 'contact' are synonymous. Each site is connected to its corresponding bond pad through a buried conductive trace. The bond pad is connected to an externalized connector through the electrode package. This makes each site individually addressable to the headstage and recording system.
On a silicon probe the sites are precisely positioned on the shank. On a thin-film grid they are precisely positioned on the face of the polyimide substrate that contacts the tissue.
From a practical standpoint, two primary properties of a site determine its recording and/or stimulation characteristics. First, its geometric area determines the tissue volume it samples, its 'region of interest'. For a silicon probe, a relatively small site (177 µm²) records spike activity of nearby neurons; a relatively large site (703 µm²) averages biopotentials over a larger area, and is well indicated for recording local field potentials. Second, its site material is a primary factor in electrical properties. NeuroNexus uses iridium for most silicon probes and platinum for most polymer grids and for the SiNAPS probes. Electro-deposited conductive coatings (such as PEDOT) can be applied to further tune electrical characteristics. These materials are well characterized and have excellent electrochemical properties and biocompatibility for neural interfacing.
Typical values
| Site area, catalog silicon probes | 92 to 1,250 µm² |
|---|---|
| Most common site area | 177 µm² (about 15 µm across), on 124 of 216 catalog probes |
| Sites per catalog silicon probe | 4 to 1,024 |
| Site metal | iridium on 209 of 216 catalog probes; platinum on 3 |